1. Field of the Invention (Technical Field)
The present invention is of differential spectral interferometry (DSI), a novel method for biomedical and materials imaging, which combines the high dynamic range of optical coherence tomography (OCT) with inherently parallel low-bandwidth image acquisition of spectral interferometry (SI). DSI efficiently removes the deleterious DC background inherent in SI measurements while maintaining the parallel nature of SI. The invention is demonstrated for both synthetic and biological samples. Because DSI is mechanically simpler than OCT, while preserving the low bandwidth, parallel nature of SI, it is competitive with OCT for biomedical applications in terms of image quality and acquisition rate.
2. Background Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
Optical coherence tomography (OCT) and spectral interferometry (SI) are low coherence optical imaging methods related to each other through the Fourier transform (FT). OCT, which takes measurements in the time domain, provides good dynamic range and image resolution. It has been demonstrated that OCT can be successfully used for sub-surface biomedical imaging at nearly video rate. A drawback of OCT is the requirement for a high-speed optical delay line in the reference arm. SI makes measurements in the frequency domain, taking advantage of inherently parallel spectral data acquisition and an optical arrangement without moving parts or high-speed modulation. Recently, Applicants reported on successful use of SI for 3D-imaging in Xenopus tadpoles. A. B. Vakhtin, et al., “Differential spectral interferometry: an imaging technique for biomedical applications”, Opt. Lett. 28,1332 (August 2003); and A. B. Vakhtin, et al. “Common path interferometer for frequency-domain optical coherence tomography,” Appl. Opt. 42, 6953 (December 2003).
The main drawback of SI is the DC character of the measurements, which results in a relatively low imaging dynamic range. A strong background associated with the spectrum of the light source itself (DC term) and the interfering waves scattered from different surfaces within the sample (autocorrelation terms) are inherently present in SI images. It is the DC and autocorrelation terms that keep the dynamic range of SI far below the theoretical shot-noise limit, thus limiting the usefulness of SI for biomedical imaging. To eliminate these two artifacts of SI, M. Wojtkowski, et al., “Complex spectral OCT in eye imaging”, Opt. Lett. 27, 1415 (August 2002), recently suggested an approach that involves complex five-frame phase and amplitude reconstruction. The method improves the signal-to-noise ratio and doubles the depth range by using both positive and negative optical path differences for imaging; however, it requires exceptional stability of the object—λ/10 within the complex-method data measurement time scale, which is 6–120 s.
A major advantage of the high-speed differential detection of the invention is that it rejects 1/f noise. In the case of 1/f noise, the magnitude of low frequency components is much larger than the magnitude of high frequency components. Thus, when the detection bandwidth allows low frequency components to pass, the noise levels are higher simply because the amplitude of the low frequency components is higher.
Therefore, it is imperative to reject low-frequency components, which is not taught by either U.S. Pat. No. 5,565,986, to knüttel or U.S. Pat. No. 6,377,349, to Fercher. While they do demonstrate that a dither in the reference arm can be added to enhance data collection and reject background terms, they do not teach or suggest that the dither and the differential data collection must be performed at high-speed. In fact, Leitgeb et al. (including Fercher) teach that simply integrating the array detector for a short period of time rejects 1/f noise. R. Leitgeb, et al., Optics Express, 11, 889–894 (21 Apr. 2003). They explicitly state that: “Fourier domain systems record one full A-scan in parallel. For short exposure times (<1 ms) also in this case 1/f noise will be neglected.” This is incorrect. To first order, a single short exposure passes all frequencies from DC up to approximately the reciprocal of the pulse width. To reject DC and low frequencies, the array must be integrated for a short period time, and the time separation between the shifted interferograms must be short, for it is this time separation—not the integration time—that determines the measurement bandwidth. In other words, synchronous detection as in the present invention must be used.